Blue Moon

All across the news and social media this week are stories about the July 31st “Blue Moon”. Let’s take a look at what is a blue moon and why it’s special.

Like the sun and stars, the moon moves across the night sky, rising the east and setting in the west. The moon has two key differences from the sun and stars:

  1. It orbits the Earth
  2. It rises and sets approximately 50 minutes later every day.

The moon does not create its own light; it is illuminated by reflected sunlight. The phases of the moon are the appearance of the illuminated portion of the moon as seen by us on Earth. Half of the moon’s surface is always bright, but the part of the illuminated portion that we can see varies. The phases of the moon move through a consistent cycle as the moon orbits the Earth and the relative positions of the Earth, moon and sun change.

New moon occurs when the moon lies between the Earth and Sun. The moon rises and sets at roughly the same time as the sun. The sun shines on the part of the moon facing it, which happens to be the side facing away from the Earth. As a result, we don’t see the moon.

Two weeks later, the sun, Earth and moon are again in a straight line. This time, the moon is opposite the sun, with the Earth in between. It rises at sunset and sets at sunrise. The illumination part of the moon’s surface is completely facing the Earth, so we see the whole thing or a full moon.

 

When the moon is new, the Earth, moon and sun are in a straight line.  However, the side of the moon illuminated by the sun is facing away from the Earth (side with the eyes in the picture).  For those on Earth, we can't see the moon.
When the moon is new, the Earth, moon and sun are in a straight line. However, the side of the moon illuminated by the sun is facing away from the Earth (side with the eyes in the picture). For those on Earth, we can’t see the moon.
During a full moon, the Earth, moon and sun are again in a line.  This time, the side of the moon illuminated by the sun is facing the Earth (the side with the eyes in the picture).  Not to scale.
During a full moon, the Earth, moon and sun are again in a line. This time, the side of the moon illuminated by the sun is facing the Earth (the side with the eyes in the picture). Not to scale.

 

 

 

 

 

 

 

Astronomers measure the moon’s phase cycle as the time from new moon to new moon or 29.53 days. The accuracy of which ancient cultures measured the lunar month varied. The Mayans charted the phases of the moon with great accuracy while others only counted the days when the moon was visible, yielding a 28 day cycle. Many cultures set their calendars based on the phases of the moon, including the Hawaiians. It’s not a coincidence that our months are roughly the same length as the lunar cycle.

Which brings us back to the blue moon… The length of our months are no longer set by the phases of the moon, rather they each have a fixed number of days. However, since the length of the month is so similar to the phases, we generally only see one full and one new moon a month. Months with a blue moon are the exception, they are months with two full moons, the second called the “blue moon”. The moon was last full on July 2nd and will be full again 29.53 days later, July 31st. A blue moon occurs once every 2.7 years.

 

So the moon won’t look blue on Friday evening. And it won’t cause anything out of the ordinary to happen. But the moon will look bright and spectacular as always

Leave a Reply

Your email address will not be published. Required fields are marked *